For short-range wireless communication, that is to say for ranges of the order of a few centimeters up to a few meters, typically around 10 cm to around 5 m, magnetic induction transmission is an attractive option. Rather than relying on an aerial to transmit an electromagnetic wave as occurs in conventional radio (which shall also be referred to hereinafter as “RF radio”) transmission, in magnetic induction transmission, a transmitter is provided with an antenna and propagates a magnetic field. The antenna is typically a coil. The receiver has as an antenna a further magnetic coil. The transmitter and receiver magnetic coils form a magnetic induction circuit, with the interlying air acting as the core. The arrangement is in some ways analogous to a conventional transformer, except to that the primary and secondary coils (transmitter and receiver antennae respectively) are not necessarily in a geometrically fixed relationship, and instead of a ferroelectric core between them, the core is air.
The coupling between the transmitter and receiver coils, and thus the strength of the received signal for a fixed power transmitted signal, depends on both the distance between the transmitter and receiver, and their relative orientation. FIG. 1 is a radial plot, showing the relative range 1 of a signal, against the angle between the transmitter and receiver coils. On the abscissa (x-axis) is shown the range when the coils are co-axial, and on the ordinate (y-axis) is shown the range, in centimeters, when the coils are co-planar.
From the figure is clear that there is a “null spot”, close to a 45° orientation between the coils. In passing, it is noted that RF radio signals suffer from similar geometrical null spots; however RF radio also suffers from fading, which generally over-rules the effect of nulls in the radiation pattern for most wireless radio applications. Fading is generally not a problem for magnetic induction radio, since the range is relatively short.
If the receiver is situated in the null spot, the received signal strength will be significantly weaker than elsewhere. For an application such as hearing aid support, in which the relative spatial arrangement between the transmitter and receiver is not known a priori, this can be a significant problem.
It is known to solve this problem by use of a technique called antenna diversity. Typically, antenna diversity is applied at the receiver side: instead of relying on a single coil, the receiver has two or more coils, arranged typically orthogonally to each other. The receiver can then switch between the multiple antennae, based on the quality of the received signal.
An alternative solution of relying on antenna diversity is known where the diversity is applied at the transmitter: in situations where there is a bidirectional link, information on the received signal quality can be returned to the transmitter, and the transmitter can then choose between which antenna to use, based on the information about the received signal quality.
Neither of these the solutions are ideal: in the case of transmitter diversity there is a requirement for a bidirectional link, which may not always be present; in the case of receiver antenna diversity, additional space to accommodate the multiple coils is required in the receiver, whereas replications which are severely space constrain, such as, for example in ear hearing aids, the additional space may not necessarily be available.
It would therefore be desirable to provide an alternative solution to the problem of nulls-spots, which does not suffer from the above limitations to the same extent.